While the characters may not look it, they are part of the same biological species. What we would call "humans" are refered to as Zeikauko (Zeh-Eeh-Kah-Ooh-Koh) or Rogteo*-Nem** (Rog-TEH-Oh), depending on what language you speak between Kolzni or Tepnem-Rho, respectively.
The scrambled phenotype happens due to ambient magical forces, or Khen-Rho, messing around with fetal development. As such, genetic inheritance doesn't play as much of a role because a lot of a person's phenotype is determined by a magical lottery that gives way for preternatural variety in the species.
There is such a thing as an average variety, in that the majority of people will still conform to a bipedal structure with two arms and a head, featuring mammal-like anatomy, somewhere between 4 and 8 feet in hight; with maybe a third of children being born who extensively break away from that model.
The Khen-Rho, however, will shy away from messing with certain components of the species. Specifically the traits that would make it impossible to survive and reproduce. That is to say, a child may be born who can live in water due to the presence of gills, but a child will not be born that NEEDS to be in water to survive as that would result in their swift demise.
Due to the mechanics of Khen-Rho, other species, what we would call animals, are more unified and easily recognizeable as a collective. The key component believed to affect a species' variety potential is attributed precocial vs altricial development.
Precocial species, the ones that are born and within hours operate as fully functional members of the species, force the ambient Khen-Rho to obey more rules so as to preserve the young's survivaility.
Altricial species, the ones that are born and must learn to exist over a much longer period of time while relying on the adults for survival, have fewer inherent rules in their development and the Khen is free to play in the sandbox.
It stands to reason that the Rogteo species is one of the most altricial, and thus have the most variety to offer.
For Zeikauko it is relatively easy to tell themselves apart from other animals by noting qualities like their ability to communicate with each other, even if they speak different languages, where trying to communicate with an animal ends up being futile for most.
Much like with humans, there is a spectrum of going from egg carriers to fertilizers, although with more variables in between.
Said variables may or may not be recognized by a given culture. Most notably, Kastina's early history involved a delicate necessity to regulate reproduction; making sure that it happened enough for a limited population to sustain itself, but also didn't happen so much that overpopulation became an issue. At this time, strict gender roles and expectations were concieved and have since prevailed even after their necessity disappeared. In most regions of Rhonum, however, the lack of any such needs yielded a culture that barely acknowledges the distinctions in the spectrum, given that it's near impossible to tell anyone's sex based on the way they look to begin with.
Reproductive compabitibilty is generally always true, although the mechanics of any given couple may not be viable according to their physical qualities. In theory, most people who can actually fit together have a higher chance of being attracted to each other, but in practice that isn't always the case. Fortunately, barring Kastina's specific historical context, this has never been an issue in terms of population growth in the world.
There is never a shortage of tailors in the world. Due to the vast variety of bodies that exist, custom tailoring is an extremely lucrative career path for just about anyone who is willing to take up the job. Many take pride in being able to provide stylish and fashionable clothes for clients that go way out of the "average" bipedal model. In a lot of cases, clothing is likely to be more of a status symbol than anything else.
However, by necessity, these cultures have very nonchalant standards of decency when it comes to clothing or lack thereof, and if an individual just finds it impractical or uncomfortable to try and clothe themselves, it is unlikely anyone would bat an eye at it. Generally speaking, so long as their privates are kept private, no one cares.
In essence, clothes serve a limited number of purposes:
1) Protection from environment.
2) Showing social status.
3) Concealing or accentuating features others will find attractive.
The basic rule of thumb is "if we can see your junk, you cover it up" and virtually anything else is fair game based on different regions and cultures.
* Speakers of Tepnem-Rho will more likely refer to themselves as Rho-Nem instead of Rogteo. This is because much of the broader levels of nomenclature in the language goes back to giving their god, Rho, ownership of everything. In that sense, Rho-Nem is colloquially used to describe the species as a people, while Rogteo is the term that more precisely identifies them as separate from other animals.
**Formally speaking, Nouns in Tepnem-Rho are identified by a Num or Nem suffix. Nem, specifically, labels the subject as a person rather than an animal or object. Most of the time, the suffix is implied and only applied when more clarity is necessary. More on this in the future when the language entries become available.